Post‐hematopoietic stem cell transplantation relapse: Role of checkpoint inhibitors

Abstract Background and Aims Despite the revolutionary effects of hematopoietic stem cell transplantation (HSCT) in treating hematological malignancies, post‐HSCT relapse is considered a critical concern of clinicians. Residual malignant cells employ many mechanisms to evade immune surveillance and survive to cause relapse after transplantation. One of the immune‐frustrating mechanisms through which malignant cells can compromise the antitumor effects is misusing the self‐limiting system of immune response by overexpressing inhibitory molecules to interact with the immune cells, leading them to so‐called “exhausted” and ineffective. Introduction of these molecules, known as immune checkpoints, and blocking them was a prodigious step to decrease the relapses. Methods Using keywords nivolumab, pembrolizumab, and ipilimumab, we investigated the literature to figure out the role of the immune checkpoints in the HSCT setting. Studies in which these agents were administrated for relapse after transplantation were reviewed. Factors such as the interval from the transplant to relapse, previous treatment history, adverse events, and the patients’ outcome were extracted. Results Here we provided a mini‐review discussing the experiences of three immune checkpoints, including nivolumab, pembrolizumab, and ipilimumab, as well as the pros and cons of using their blockers in relapse control after HSCT. In conclusion, it seems that CI therapy seems effective for this population. Future investigations may provide detailed outlook of this curative options.

donor lymphocyte infusion (DLI) and donor-derived T/natural killer (NK) cell infusion, could not succeed as much as expected due to the potential complications and risks. 3 DLI is often limited to chronic myeloid leukemia (CML) patients due to the increasing high risk of GVHD in others 4 ; NK cell therapy is still hesitating concerning difficulties in its expansion and standardization protocol. Therefore, a thoughtful choice of alternative treatment for post-HSCT relapse deserves more investigations. 4 Survived leukemic stem cells (LSCs) from conditioning regimens and poor immune reconstitution are responsible for relapse after HSCT. 5 There is evidence that niche interactions between bone marrow stroma and LSCs provide protective properties against chemotherapy. 6 Also, one of the reasons for the resistance of LSCs is their dormant status. 7 Releasing exosomes and microvesicles, LSCs alter the healthy niche to leukemic-supportive microenvironment. 8 Thanks to the favorable effect of graft-versus-leukemia (GVL), the relapse rate is lower in allo-HSCT than autologous HSCT (auto-HSCT). Therefore, relapse may be a hint of the failure of GVL. In other words, mechanisms that increase the GVL effect can also increase GVHD, which explains why less relapse occurs in patients with a history of mild GVHD.
Immune cells have a substantial role in relapse control. Reduced dendritic cells (DCs) have been associated with relapse after HSCT. 9 Although regulatory T cells (T regs) effectively prevent GVHD, they may prohibit GVL. 10 Consistent with this, it has been demonstrated in a study that a higher dose of CD34+ cells correlates with a higher rate of relapse, which proposed that it could be due to more malignant cells and T regs, decreasing antileukemic effects. 11 Because of a higher percentage of T cells in peripheral blood HSCT, the relapse rate is lower at the expense of a higher incidence of GVHD. 12 NK cells can also be considered as another critical relapse-inhibiting agent because relapse in high-risk acute myeloid leukemia (AML) patients transplanted by killer immunoglobulin-like receptor (KIR)ligand mismatch was 0%. 13 Similarly, a study conducted by Willem et al. 14 demonstrated that KIR/human leukocyte antigen (HLA) incompatibility has favorable effects in reducing relapse.
Sometimes, despite the sufficient number, lymphocytes are not functional due to the inhibitory interaction between leukemic and immune cells. Impaired function of B and T lymphocytes is associated with relapse after HSCT. In patients who relapsed after HSCT, bone marrow-infiltrating CD8+ T cells have a unique immunophenotype signature called "exhausted," which expresses a high level of programmed cell death protein 1 (PD1), cytotoxic T-lymphocyteassociated protein 4 (CTLA4), and/or other immune-checkpoints (CI).
The presence of exhausted CD8+ T cells and the immunosuppressive state of the niche in relapsed myeloma reveal the potential of immunotherapy in preventing disease recurrence. 15 Regarding the checkpoint's role in various malignancies, checkpoint inhibitors (CIs) have been introduced for disease treatment and management. Evaluation of CIs in some malignancies displayed satisfactory results, especially in melanoma and lung cancers. In hematological disorders, there are growing studies aiming at the effect of CIs in relapse management. Here we reviewed the role of CIs in the HSCT setting.

| PD1 BLOCKADE IN POST-HSCT RELAPSE
Recently, the application of PD1 inhibitors in hematological malignancies has been strikingly growing. Since the Hodgkin lymphoma (HL) microenvironment demonstrates unique features, it is potentiated to get the advantage of CIs. There are a few Red-Sternberg cells and numerous ineffective immune cells in the lymphoma niche. 16 Therefore, most of the investigations are focused on refractory/relapsed HD (R/R HD).
It has been reported that the expression of inhibitory molecules, such as PD1 on the T-cell surface is associated with relapse after HSCT, independent of GVHD. The predictive value of PD1-high immune cells for leukemia relapse post-HSCT has also been suggested. 15  experience, 21 in which attenuated GVL is associated with increased expression of PD1 in the allogeneic recipients. They reported that delayed adoptive transfer needs PD1/PDL1 inhibition to promote GVL effects. GVL reviving could occur by PDL1 inhibition without causing GVHD. In contrast, coadministration of PDL1 blockers with the early adoptive transfer may induce fatal GVHD, which is the main concern in using these drugs. 22 Given the fact that the antitumor effects of GVL have pertained to the donor T cells, it has been concluded that PD-1 blockade could induce remission in a patient unresponsive to previous DLI therapy. 23 On the other hand, a recent study suggested that although CI therapy plus DLI reduce post-HSCT relapse, it also induces GVHD potential in many patients, highlighting the importance of patient selection and appropriate timing of the CI administration. 24 Thus, CI therapy alone may have more acceptable results than combining with DLI in some cases. Two prominent PD1 inhibitors, including nivolumab and pembrolizumab, have been investigated in transplantation studies.
Here, we reviewed the literature on these CIs.

| NIVOLUMAB AND STEM CELL TRANSPLANTATION
Nivolumab, a humanized IgG4 kappa monoclonal antibody, has been approved for relapse treatment after auto-SCT since 2016. 25 There are many promising results about using nivolumab in post-HSCT relapse. One prominent example was a patient with Hodgkin disease who experienced chemotherapy and relapsed after auto-HSCT. Using a PD1 blocker, 32 months after allo-HSCT, he achieved complete remission (CR). 26 (Table 1).

| Introduction and approval Hodgkin lymphoma
Another PD1 inhibitor agent is pembrolizumab which is a humanized IgG4 antibody. Pembrolizumab is approved for relapsed or refractory classical Hodgkin lymphoma in 2020. Clinical trials approved administration of 200 mg every 3 weeks is effective for relapsed Hodgkin lymphoma. 65 The results of clinical trials led to the approval of this new agent. 66 An analysis conducted by Armand et al. demonstrated that receiving pembrolizumab after SCT seems beneficial with a 73% of overall response rate (ORR), 59% partial remission, and 14% complete remission. They also could show that this medication induces upregulation of IFN-gamma. 67 In a report of two cases of HL after allo-SCT, safe and successful treatment was obtained by achieving partial and complete remission. 68 Chan et al. shared their results on using CI agents before auto-SCT. Evaluation of pembrolizumab therapy in 5 R/R HL patients with a history of failure in different treatment strategies showed 100% ORR and almost no toxicity was observed. 69 Strengthening the immune system is always likely to cause unwanted side effects, as some degree of autoimmunity, which can sometimes be a disaster for the patient. The first fatal GVHD after pembrolizumab treatment was reported in 2016.
The patient died due to skin and lung chronic GVHD. 70 Another evidence is a report of post-pembrolizumab insulin-dependent diabetes in a 12-year-old boy with a history of disease recurrence after auto-SCT. Getting rid of lymphoma came at the cost of developing diabetes for this patient, highlighting the importance of cautiousness when applying CI agents in the pediatric setting. 71 In this regard, the findings of a study that revealed two death out of three indicate the importance of dose optimizing in preventing complications. 72  are not very satisfactory. 68 Promising results have been detected in the anaplastic large-cell lymphoma patients. A study reported that a relapsed patient with the previous auto-and allo-SCT was asymptomatic after receiving pembrolizumab. Though, it should be noted that she experienced hepatic GVHD. 73 In the case of NK/T lymphoma, the patient who received pembrolizumab before manipulated haploidentical transplantation achieved an almost complete response. 74 ROSHANDEL ET AL. Ipilimumab, a fully humanized IgG1 monoclonal antibody against CTLA4, is the first and only FDA-approved CTLA4 inhibitor for metastatic melanoma. Ipilimumab has been used for many hematological disorders after relapse. In recent years, ipilimumab has also been tried for some post-SCT relapsed hematological disorders. 82 Accordingly, Bashey et al. reported organ-specific immune adverse events concurrent with tumor regression in 14% of cases. However, no acute or chronic GVHD was observed. 83 Later, Davids et al. 84 proposed the late administration of ipilimumab in post-HSCT relapse for more beneficial responses. However, it should be noted that the absence of severe GVHD was one of the inclusion criteria for both of these two studies. Therefore, the previous complications of patients must be considered before patient selection for this treatment strategy. Another study has investigated ipilimumab in combination with lenalidomide for patients who relapsed after allogeneic and autologous HSCT. The response rate was higher than the expectations for allo-HSCT patients, probably due to lenalidomide addition. 85 Another evidence is extracted from a report by Gros  The association between BTLA and GVHD has been discussed in preclinical investigations and showed that, in the first days after HSCT, the anti-BTLA antibody could control GVHD, maintain the GVL effect through deactivating donor T cells, and cause a relative increase in natural regulatory T cells. 89 Contrarily, del Rio et al. 90 suggested that blocking BTLA/HVEM pathway is insufficient to in-

| COMPLICATIONS OF CI THERAPY IN HSCT
A meta-analysis demonstrated rare fatal toxic effects due to CI therapy. The highest rate of complications was reported when PD1/ PDL1 and CTLA4 inhibitors were administered together, which only occurred in 1.23% of patients. The most involved organs were the colon, liver, lungs, pituitary, thyroid, and skin. 91 Kaloyannidis et al.
reported that a patient who received nivolumab before transplantation showed de novo Psoriasis Vulgaris, which resolved after auto-HSCT. It has been presumed that conditioning regimens (Melfalan) and auto-HSCT could eliminate skin complications. 92

ACKNOWLEDGMENT
The authors would like to thank the colleagues in the Hematopoietic Stem Cell Transplantation center.

CONFLICTS OF INTEREST
The authors declare no conflicts of interest.